1. Field of the Invention
The present invention relates in general to internal combustion engines and more particularly to an arrangement of a pressure sensor for detecting the pressure of burned gases within a cylinder of an internal combustion engine.
2. Description of the Prior Art
For the purpose of improving the fuel consumption and output of an internal combustion engine, it is quite useful to know the actual combustion state of a mixture within a cylinder. For example, by feedback controlling an ignition timing in accordance with the combustion state of a mixture, an optimal fuel consumption and engine output can be attained through prevention of detonation.
As a measure for knowing such a combustion state, it has been commonly practiced to detect the pressure of burned gases within an engine cylinder, i.e. a combustion pressure. To this end, such a measure as shown in FIGS. 8 through 10 is known in the art.
Referring to the figures, the reference numeral 10 indicates a cylinder head having a threaded hole 11 into which an ignition plug 12 is screwed. Upon attachment of the ignition plug 12 into the threaded hole 11, an annular pressure sensor 15 is disposed between the ignition plug 12 and a plug seating 14 to be clamped therebetween.
The pressure sensor 15 has a laminated structure and includes an annular central electrode 16 at a central part thereof, a pair of annular piezoelectric elements 17 on the opposite sides of the central electrode 16, an upper electrode 18 and lower electrode 19 placed on the opposite outer surfaces of the piezoelectric elements 17 and a mold member 20 made of an electrically non-conductive material and covering the inner and outer peripheries of the electrodes and elements to join the same together.
The central electrode 16 has a lead-out portion extending through the mold member 20 to be electrically connected to a lead wire 21.
The pressure sensor 15 is adapted to serve as a washer for the ignition plug 12 and is held in a compressed state. The fastening force with which the ignition plug 12 is fastened to the cylinder head 10 varies as the ignition plug 12 is subjected to various combustion pressures within the cylinder, thus causing the voltage developed by the piezoelectric elements 17 to vary in accordance with variations of combustion pressure.
In the above manner, combustion pressures of the internal combustion engine can be accurately converted into electric signals to be measured.
A disadvantage of the above mentioned prior art arrangement is that there is a difficulty in bringing the lower electrode 19 into contact with the plug seating 14 perfectly or desirably. This is particularly true when the hole at the bottom of which the seating 14 is provided is deep.
Another disadvantage is that upon attachment of the ignition plug 12, the pressure 15 tends to rotate to damage the lead wire 21.
A further disadvantage is that upon removal of the ignition plug 12 the pressure sensor 15 is likely to be removed together with the ignition plug 12.
A yet further disadvantage is that the lead wire 21 tends to pick up ignition noise, causing a hindrance to accurate measurement of the combustion pressure.